Liquid metal alloy energy storage device

ABSTRACT

An energy storage device configured to exchange energy with an external device includes a container having walls, a lid covering the container and having a safety pressure valve, a negative electrode disposed away from the walls of the container, a positive electrode in contact with at least a portion of the walls of the container, and an electrolyte contacting the negative electrode and the positive electrode at respective electrode/electrolyte interfaces. The negative electrode, the positive electrode and the electrolyte include separate liquid materials within the container at an operating temperature of the battery.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/839,130, filed Jul. 19, 2010, now U.S. Pat. No. 9,076,996,which is a continuation-in-part of U.S. patent application Ser. No.12/505,937, filed Jul. 20, 2009, the disclosures of which areincorporated by reference herein in their entirety.

TECHNICAL FIELD

This invention relates to electrical energy storage. It relatesespecially to electrochemical energy storage cell devices or batterieshaving liquid components and enhanced current-carrying capabilities.

BACKGROUND

Balancing supply and demand of electrical energy over time and locationis a longstanding problem in an array of applications from commercialgenerator to consumer. The supply-demand mismatch causes systemic strainthat reduces the dependability of the supply, inconveniencing consumersand causing loss of revenue. Since most electrical energy generation inthe United States relies on the combustion of fossil fuels, suboptimalmanagement of electrical energy also contributes to excessive emissionsof pollutants and greenhouse gases. Renewable energy sources like windand solar power may also be out of sync with demand since they areactive only intermittently. This mismatch limits the scale of theirdeployment. Large-scale energy storage may be used to support commercialelectrical energy management by mitigating supply-demand mismatch forboth conventional and renewable power sources.

One approach to energy storage is based on electrochemistry.Conventional lead-acid batteries, the cheapest commercial batterytechnology on the market, have long been used for large-scaleelectrochemical energy storage. Facilities housing vast arrays oflead-acid cells have been used to provide high-capacity electricitystorage, on the order of 10 MW. However these facilities are neithercompact nor flexibly located. The short cycle life of lead-acidbatteries, on the order of several hundred charge-discharge cycles,limits their performance in uses involving frequent activation over awide voltage range, such as daily power management. The batteries do notrespond well to fast or deep charging or discharging, which lowers theirefficiency and reduces their lifespan.

Sodium-sulfur (“NAS”) batteries have been adapted to large-scale powermanagement facilities in the US and Japan. An NAS battery incorporatesmolten sodium and sulfur electrodes opposed across a solid ceramicelectrolyte. The electrolyte must be very thin in order to maximizesodium ion conduction, but this makes it mechanically fragile andimposes severe limits on the maximum size of an individual cell. This,in turn, affects scalability, i.e., large capacity must be achievedthrough many small cells rather than through few large cells, whichgreatly increases complexity and ultimately increases the cost of thesystem. Cell construction is complication by sodium's violent reactionwith water and rapid oxidation in air.

There is, accordingly, a need for an energy storage device combiningcapacity, economy, flexibility and long life.

SUMMARY OF THE INVENTION

In one embodiment, an electrochemical battery comprises a container, apositive electrode, a negative electrode and an electrolyte, disposedbetween the positive electrode and the negative electrode, all existingas respective liquid material layers in a vertical stack in thecontainer at the operating temperature of the battery so that adjacentlayers form respective electrode/electrolyte interfaces. The batteryalso comprises a circulation producer configured to generate circulationwithin one of the layers, thereby inducing a flow of liquid material ofthe one of the layers to and from one of the electrode/electrolyteinterfaces.

In another embodiment, an electrochemical battery configured forexchanging energy with an external device comprises an open topcontainer having walls and containing a positive electrode, a negativeelectrode and an intervening electrolyte. The electrodes and theelectrolyte exist as liquid material layers within the walls of thecontainer at the operating temperature of the battery, with one of thepositive electrode and the negative electrode being disposed over theelectrolyte. A lid closes the top of the container. A positive currentcollector is in electrical contact with the positive electrode. Anegative current collector is in electrical contact with the negativeelectrode. The positive current collector and the negative currentcollector are adapted for connection to the external device to create acircuit through which current flows, and the current collector incontact with the electrode disposed over the electrolyte is suspendedfrom the lid and comprises a composite electrically conductivestructure. The structure includes a first member that holds theelectrode disposed over the electrolyte spaced away from the walls andis of a first substance that is not wet by the liquid material of saidone electrode; and a second, electrically conductive member within thefirst member that is of a second substance that is wet by the liquidmaterial of said one electrode.

In another embodiment a method of exchanging energy with an externaldevice comprises providing an external energy exchanging device and abattery. The battery includes a container containing a positiveelectrode, a negative electrode and an intervening electrolyte, thepositive and negative electrodes and the electrolyte existing as liquidmaterial layers in a vertical stack in the container so that adjacentlayers form respective electrode/electrolyte interfaces; a positivecurrent collector in electrical contact with the positive electrode; anegative current collector in electrical contact with the negativeelectrode; and electrical connections connecting the external energyexchanging device to the positive and negative current collectors,thereby creating a circuit through which current flows. The method usesnormal operational energy in the battery to generate circulation withinat least one of the layers so as to increase the flux of material of theat least one of the layers to and from one of the electrode/electrolyteinterfaces.

In yet another embodiment, an electrochemical battery is configured toexchange energy with an external device. The battery comprises anelectronically conductive molten positive electrode comprising analkaline earth metal and an additional element; an electronicallyconductive liquid negative electrode comprising the alkaline earthmetal; and a liquid electrolyte comprising cations of the alkaline earthmetal, disposed between the positive electrode and the negativeelectrode to form respective electrolyte-electrode interfaces therewith.The positive electrode, the negative electrode and the liquidelectrolyte exist as respective liquid layers of respective liquidmaterials in a vertical stack, and the alkaline earth metal is presentin respective disparate chemical potentials in the positive electrodeand the negative electrode, thereby originating a voltage therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings,wherein identical reference numerals designate analogous functionalelements, and in which:

The invention description below refers to the accompanying drawings,wherein identical reference numerals designate analogous functionalelements, and in which:

FIG. 1 is a vertical section showing a self-segregating alkaline earthmetal-ion energy storage battery constructed in accordance with theinvention;

FIGS. 2A-2C are vertical sections illustrating the charging process of aself-segregating alkaline earth metal-ion energy storage battery unitconstructed in accordance with the invention;

FIGS. 3A-3C are vertical sections illustrating the discharging processof a self-segregating alkaline earth metal-ion energy storage batteryunit constructed in accordance with the invention;

FIG. 4 is a vertical section showing another embodiment of theself-segregating alkaline earth metal-ion energy storage battery unitconstructed in accordance with the invention;

FIGS. 5A-5B are vertical sections illustrating the charging process of abattery, having a liquid metal negative electrode held by a suspendedstructure, constructed in accordance with the invention;

FIG. 6A is a vertical section illustrating a battery, having a liquidnegative electrode held by a suspended structure, constructed inaccordance with the invention and FIGS. 6B-6C are vertical sections, ona larger scale, of alternative negative current collectors suitable forthe device shown in FIG. 6A;

FIG. 7 is a vertical section illustrating a liquid-layer batteryconstructed in accordance with the invention, having a porous electrodeseparator;

FIGS. 8-14 are vertical sections of battery embodiments, constructed inaccordance with the invention, wherein one or more free convection cellsare promoted in at least one of the liquid constituents thereof by acirculation producer comprising different thermal management devices;

FIGS. 15-18 are vertical sections of battery embodiments, constructed inaccordance with the invention, wherein one or more circulation cells areinduced in at least one of the liquid constituents thereof by acirculation producer comprising different magnetic induction devices;

FIG. 19 is a perspective view showing a single alkaline earth metal ionenergy storage battery unit constructed in accordance with theinvention;

FIG. 20 is a perspective view showing a linear assembly of four batteryunits; and

FIG. 21 is a perspective view showing a 16-unit array.

Features in the drawings are not necessarily to scale.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It will be understood that as used herein, “battery” may encompassindividual electrochemical cells or cell units, comprising a positiveelectrode, a negative electrode and an electrolyte, and configurationscomprising a plurality of electrochemical cells. With reference to FIG.1, an alkaline earth metal ion energy storage cell, or battery,indicated generally at 10, incorporates three distinct liquidconstituents: a molten metal body 14 that serves as negative electrode,also referred to as the active metal electrode; an electronicallyconductive multi-elemental liquid body 16 that serves as positiveelectrode, also referred to as the alloy electrode; and an interveningionically conductive electrolyte 20.

The electrically conductive liquid layers 14, 16 and 20 are confined inan electronically conductive container 22 which illustratively providesmechanical support to an insulating inner sheath 24. The sheath 24prevents shorting by electronic conduction between the negativeelectrode 14 and the positive electrode 16 through the container 22.

The container 22 is covered by a lid 26 which is illustrativelyelectronically conductive. An electrically insulating seal 29electrically isolates the lid 26 from the container 22 and confinesmolten constituents and vapors within the container 22. A portion of thelid 26 in contact with the negative electrode 14 functions as a negativecurrent collector 27, through which electrons may pass to an externalsource or sink (not shown) by way of a negative terminal 28 in contactwith the lid 26. A portion of the container 22 in contact with thepositive electrode 16 functions as the positive current collector 23 ofthe battery 10, through which electrons may pass to the external sourceor sink by way of a positive terminal 30 connected to the container 22.The placement of the negative terminal 28 and the positive terminal 30may facilitate arranging individual cell units in series by connectingthe negative terminal 28 of one cell unit to the positive terminal 30 ofanother cell unit 10 to form a larger battery.

An inert gas layer 32 overlaying the negative electrode 14 mayaccommodate global volume changes in the three-phase system of thebattery 10 during charging and discharging thereof or due to temperaturechanges. Optionally, the lid 26 or seal 29 incorporates a safetypressure valve (not shown).

The container 22 and the lid 26 are each of a material having therequisite electronic conductivity, mechanical strength, and resistanceto chemical attack by the liquid electrodes 14 and 16 and electrolyte20. The sheath 24 is of an electronically insulating material and may becorrosion-resistant against the two liquid electrodes 14 and 16 and themolten electrolyte 20. Boron nitride, aluminum nitride, alumina, andmagnesia are candidate sheath materials. The seal 29 may be formed ofone or more materials such as magnesia cement, aluminoborate glasses,and other high temperature sealants as known to those skilled in theart.

The electrodes 14 and 16 and electrolyte 20 are constituted to establishchemical and physical properties compatible with simplicity and economyof construction, robustness, and rapid and efficient receipt anddelivery of electrical energy. The use of electronically conductiveliquids for electrodes 14 and 16 with a liquid electrolyte 20facilitates facile oxidation and reduction of the active alkaline earthmetal and its cation at the electrodes 14 and 16. The electronicconductivity of the liquid electrodes promotes high current densityduring operation of the cell 10 by enabling electron-transfer reactionsto occur at sites over entire liquid electrode-electrolyte interfacesrather than being limited to triple-phase intersections. Furthermore,because reactions at both electrodes occur entirely in the liquid state,the reaction kinetics are not throttled by the nucleation of distinctproduct phases. Thus, the constituents of the cell 10 are consistentwith extremely high current densities on the order of 1 A/cm², amagnitude observed in the high-temperature electrometallurgicalindustry, e.g., in the electrolytic production of aluminum.

The chemical compositions of the molten electrodes 14 and 16 areformulated conjunctionally to incorporate an active alkaline earthmetal, such as beryllium, magnesium, calcium, strontium or barium atrespective disparate thermodynamic activities, thereby generatingvoltage between the electrodes 14 and 16. In order to createthermodynamic activity disparity of the active alkaline earth metalbetween the negative 14 and positive 16 electrodes, at least one of theelectrodes 14 and 16 includes one or more additional elements, otherthan the alkaline earth metal. Any additional element may be, e.g.,miscible in the liquid composition of the electrode 14 or 16 so as toform a liquid alloy with the alkaline earth metal, or exist in acompound with the alkaline earth metal under the operating conditions.The one or more additional elements are chosen to constitute thepositive electrode 16 as an environment of relatively low thermodynamicactivity of the active alkaline earth metal, compared to the negativeelectrode 14, when the cell 10 is in a charged state. As used hereinwith reference to the positive alloy 16, “alloy electrode” does notencompass only liquid-phase solutions conventionally referred to asalloys but also liquid-phase compounds of the active alkaline earthmetal and one or more additional elements.

In choosing additional elements, in additional to the active alkalineearth metal, for the electrodes 14 and 16, not only chemical equilibriaand solution thermodynamics in the electrodes 14 and 16 but also theirinteractions with the electrolyte 20 must be considered, as well astheir relative densities and liquid ranges. Any element in theelectrodes 14 or 16 in addition to the active alkaline earth metalideally should not interact with the ions in the electrolyte in a waythat would provide a competing pathway for charge transport andcircumvent the prescribed electrode reactions.

Thus, elements that may be appropriate for incorporation in the alloyelectrode 16 to reduce the activity of the active metal may includealuminum, tin, lead, germanium, indium, pnicogens such as bismuth andantimony, and chalcogens such as tellurium and selenium. The electrodes14 and 16 may include other species, for example, to tailor physicalproperties or enable electrochemical monitoring of the extent ofdischarge, as is known to those skilled in the art. For example, one ormore additional transition metals or metalloids, such as copper,silicon, iron, or gallium, may be added in smaller quantities adjust thedensity and/or melting point.

The use of an alkaline earth metal, such as beryllium, magnesium,calcium, strontium or barium, in the electrodes 14 and 16 of theall-liquid alkaline earth metal ion energy storage batteries 10 may haveseveral advantages over conventional battery materials. For example, thevoltage generated by the illustrative calcium-metalloid couple in asingle cell may be on the order of 0.5 V, 0.75 V or greater, exceedingthe corresponding voltage of an analogous lithium- or sodium-basedsystem and correlating with a larger energy capacity on a molar basis.Also, calcium and magnesium, for example, are relatively inexpensivecompared to lead or alkali metals and are easier to manage than alkalimetals in that they may be safely handled in open air, do not reactviolently with water, and can be held with bare hands. Whereas an alkalimetal cation carries a single positive charge, an alkaline earth metalcation carries a +2 charge and consequently makes available in theory adoubled charge capacity of the alkaline earth metal ion energy storagecell 10 compared to alkali metal cells.

The electrolyte 20 of the battery 10 may be a molten salt, dissolving acation of the active alkaline earth metal, also referred to herein asthe active cation, and one or more supporting compounds. The electricalconductivity of the electrolyte 20 may be greater than 0.01 siemens/cm,0.05 siemens/cm or a greater value.

Illustratively the molten salt is a chloride, such as a chloride of theactive alkaline earth metal. Alternatively, the salt of the activealkaline earth metal may be, e.g., a non-chloride halide, abistriflimide, fluorosulfano-amine, perchlorate, hexaflourophosphate,tetrafluoroborate, carbonate or hydroxide. A supporting compound istypically added to enhance ionic conductivity, and/or to inhibitelectronic conductivity through the electrolyte. The supportingelectrolyte may comprise any of the aforementioned anions and a cationsuch as an alkali or alkaline-earth metal, an imide, amine, ammonium,phosphonium or pyrrolidinium.

Other additives to the electrolyte 20 may reduce the viscosity, depressthe melting point, alter the density, or reduce vapor pressure. Thesupporting electrolyte and any other additives illustratively have freeenergies of formation more negative than that of the reaction compoundso that the cationic constituents of the supporting electrolyte and anyadditive electrodeposit at more extreme values of potential, or athigher values of cell voltage, than that associated with moving theactive alkaline earth metal from the active metal electrode 14 to thealloy electrode 16, in order to limit the electrode reactions to theoxidation and reduction of the active alkaline earth metal. These andother considerations informing the choice of electrolyte composition areknown to those skilled in the art.

If the active alkaline earth metal is calcium, the electrolyte 20 mayfurther include complexing ligands to reduce the solubility of elementalcalcium in molten calcium chloride. Ligands delivered by largemonovalent cations having a relatively low charge density may complexdivalent cations such Ca²⁺. For example, chloride anions introduced byaddition of potassium chloride, sodium chloride, or other appropriatealkali metal-halide salts may lower the solubility of calcium metal in acalcium-halide mixture. Electrolyte compositions in the systemKCl—KI—KBr—CaCl₂, at 5 mol % to 50 mol % CaCl₂, may provide the desiredcombination of ionic conductivity, melting temperature and complexingaction.

The compositions of the electrode 14 and 16 and electrolyte 20 may beformulated so that all-liquid operation occurs at moderately elevatedtemperatures, illustratively between 300° C. or 400° C. and 750° C.Operation at temperatures greater than about, e.g., 300° C. or 400° C.,facilitates electrode reaction kinetics and ion migration in theelectrolyte 20. However, difficulties such as volatilization of cellconstituents, structural weakness, chemical attack of ancillarymaterials, and power required to maintain liquidity of the electrodes 14and 16 and electrolyte 20 become more likely as operating temperatureincreases. Operation below 750° C. may afford the kinetic advantages ofhigh temperatures without the associated drawbacks.

The electrodes 14 and 16 and the electrolyte 20 may be furthermoreformulated so that their densities are ordered in accordance with theirfunctions in the battery 10. Embodiments having respective densitiesincreasing, as shown in FIG. 1, or decreasing in the order negativeelectrode 14/electrolyte 20/positive electrode 16 may spontaneouslyself-segregate into the illustrated vertically stacked layered structureupon melting, providing for simple manufacture from billets.

Energy storage in the alkaline earth metal ion battery 10 is not limitedto any particular method of attaining or maintaining the operatingtemperature thereof. The constituents forming any of the layers 14, 16,and 20 may be melted in a separate heated chamber with sufficientsuperheat to allow transfer to the container 22. In another approach,external heaters (not shown) placed, for example, within the wall of thecontainer 22 may be used before or during operation. Alternatively, thebattery 10 may be self-heating during operation through appliedoverpotentials. Techniques for achieving and managing temperatureprofiles in molten constituents, and other practical aspects ofelectrometallurgical systems potentially helpful to implementing powerstorage using liquid alkaline earth metal electrodes, such asconstruction of apparatus for use with molten salts and liquid metals,are known to those skilled in the art and have been described, forexample, in commonly owned pending U.S. application Ser. No. 11/839,413,filed Aug. 15, 2007 and Ser. No. 12/505,937, filed Jul. 20, 2009 and inU.S. Pat. Nos. 4,999,097 and 5,185,068, the entire disclosures of all ofwhich are incorporated herein by reference.

The illustrative alkaline earth metal ion battery 10 receives ordelivers energy by transporting an alkaline earth metal, referred toherein as the active alkaline earth metal, between the two moltenelectronically conductive electrodes 14 and 16 via an electrochemicalpathway. The liquid electrolyte 20 comprising a cation of the activealkaline earth metal enables ionic transport of the active alkalineearth metal during charging or discharging.

FIGS. 2A-2C illustrate the function of the cell 10 during charging. FIG.2A shows the cell 10 in an uncharged or discharged state. Beforecharging, the positive electrode 16 contains atoms of the activealkaline earth metal. The negative electrode 14 meets the electrolyte 20at an active metal-electrolyte interface 42. The positive electrode 16meets the electrolyte 20 at an alloy-electrolyte interface 46.

With reference to FIG. 2B, to initiate charging, the terminals 28 and 30are connected to an external charging circuit 48 driving transport ofthe active alkaline earth metal from the positive electrode 16, throughthe electrolyte 20 to neutral metal at a higher chemical potential inthe negative electrode 14. During charging, electron current travelsfrom the external circuit through the negative current collector 27 intothe negative electrode 14 and to the active metal-electrolyte interface42. Active cations M²⁺ move across the electrolyte 20 toward the activemetal-electrolyte interface 42. The active cations and the electronsmeet at the interface 42 and are consumed in the reduction half-cellreaction M²⁺+2e⁻→M. The neutral active alkaline earth metal atoms Mcreated in the half-cell reaction accrue to the negative electrode 14.As the active alkaline earth metal M accumulates in the negativeelectrode 14, the active metal-electrolyte interface 42 moves furtheraway from the negative current collector 27. At the alloy-electrolyteinterface 46 atoms of the active alkaline earth metal M in the positiveelectrode are oxidized in the half-cell reaction M→M²⁺+2e⁻. As activecations M²⁺ enter the electrolyte 20, electrons are freed to passthrough the positive current collector 23 to the external chargingcircuit 48. Oxidation of the active alkaline earth metal atoms M shrinksthe positive electrode 16, and the alloy-electrolyte interface 46 movestoward the positive current collector 23.

FIG. 2C shows the battery 10 in its final charged state. Charging haschanged the composition of at least the positive electrode 16 by loss ofatoms of the active alkaline earth metal. The alloy electrode 16 may inprinciple be nominally free of the active alkaline earth metal, andtherefore not actually be an alloy, mixture or compound at this point inthe charge-discharge cycle. The thickness of the negative electrode 14has grown at the expense of the positive electrode 16. Since thecharging process is conservative with respect to the active cations, thethickness of the electrolyte 20 is ideally unchanged.

The active alkaline earth metal deposited in the molten active metalelectrode 14 represents stored electrical energy which may persistindefinitely, as long as no external electronic path joins the twoelectrodes 14 and 16. The half-cell reactions in the cell 10 generateliquid-phase products that remain at the electrodes 14 and 16, incontact with the electrolyte. While the electrodes 14 and 16 andelectrolyte 20 are at a liquid range temperature, the active alkalineearth metal and the active cation remain available to mechanizedischarge via an electrochemical pathway. This reversibility suits theactive alkaline earth metal ion batteries for energy storage.

FIGS. 3A-3C illustrate discharging the battery 10. FIG. 3A shows thecell 10 in a charged state. With reference to FIG. 3B, connecting theterminals 28 and 30 to an external load 49 initiates discharge. Duringdischarge the active alkaline earth metal moves spontaneously from thenegative electrode 14, through the electrolyte 20 as active cations, andreverts to neutral metal at a lower chemical potential in the positiveelectrode 16. Electron current travels into the cell through thepositive current collector 23 and the positive electrode 16 to thealloy-electrolyte interface 46. Active cations M²⁺ migrate across theelectrolyte 20 toward the alloy-electrolyte interface 46. Active cationsM²⁺ and electrons are consumed at the interface 46 in the reductionhalf-cell reaction M²⁺+2e⁻→M. The neutral active alkaline earth metalatoms M produced accrue to the positive electrode 16. As the activealkaline earth metal M accumulates in the positive electrode 16, thealloy-electrolyte interface 46 moves further away from the positivecurrent collector 23. At the active metal-electrolyte interface 42,atoms of the active alkaline earth metal M in the negative electrode 14are oxidized in the half-cell reaction M→M²⁺+2e⁻. The active cations M²⁺produced enter the electrolyte 20, and the freed electrons pass throughthe negative current collector 27 to the external load 49. Oxidation ofthe active alkaline earth metal atoms causes attrition of the negativeelectrode 14, with movement of the active metal-electrolyte interface 42toward the negative current collector 27.

FIG. 3C shows the cell 10 in its final discharged state. Charging haschanged the composition of at least the positive electrode 16 due toaccretion of active alkaline earth metal atoms. The thickness of thepositive electrode 16 has grown at the expense of the negative electrode14. Since the discharging process is conservative with respect to theactive cations, ideally the thickness of the electrolyte 20 isunchanged. The substantially constant thickness of the electrolyte layerthroughout the charge-discharge cycle enables the use of an electrolytelayer that is relatively thin compared to the electrode bodies. The thinelectrolyte layer, combined with the inherently low resistivity ofmolten halides, minimizes the ohmic overpotential associated with theelectrolyte. The energy capacity of the cell 10, which is no greaterthan the smaller of the quantities of active alkaline earth metal thatcan be accommodated by the negative electrode 14 and by the positiveelectrode 16, respectively, can be augmented by increasing the quantityof material in the electrodes 14 and 16 without, in principle,increasing the mass of the electrolyte 20 or its associated IR drop. Forexample, the thickness of the electrolyte 20 may be on the order of only10%, 20% or 50% of the thickness of either of the electrodes 14 and 16.

In an illustrative embodiment, referred to herein as a calcium-bismuthbattery, the active alkaline earth metal of the battery 10 is calcium(ρ_(liquid)≈1.4 g/ml), and an additional element diluting calciumactivity in the alloy electrode 16 is bismuth (ρ=9.8 g/ml, T_(m)=271°C.). The electrolyte 20 is based on, e.g., the KCl—CaCl₂ eutectic(T_(m)=600° C.) at 25 mol % CaCl₂ with 10 mol % KI added to increasedensity. The liquid densities of KCl, CaCl₂, and KI are 1.5 g/ml, 2.07g/ml, and 2.33 g/ml, respectively. The operating temperature of the cell10 is illustratively about 700° C. The container 22 and lid 26 areillustratively of mild steel.

In addition to calcium, the illustrative active metal electrode 14 maycomprise magnesium, so that the liquid range of the electrode 14 in theembodiment is in the moderately elevated temperature range, lower thanthe melting point of calcium (850° C.). Diluting the calcium in theactive metal electrode 14 necessarily reduces the activity of calcium inthe electrode 14, thereby reducing the voltage deliverable by thebattery 10. A relatively marked reduction in voltage is to be expectedwhen the resulting system, like the calcium-magnesium binary system,forms compounds in the solid state, indicative of a negative deviationfrom ideality. It has been discovered that it is possible to includeanother metal, for example another alkaline earth metal, in addition tothe active alkaline earth metal, in the electrode 14 in sufficientquantity to bring the operating temperature into the desired moderatelyelevated range without unacceptable compromise of the cell voltage. Forexample, adding magnesium to a concentration of 80 atomic percent maygive the active metal electrode 14 a melting temperature less than 700°C. while only diminishing the voltage of the calcium ion cell by about0.1 V. The calcium concentration in the active metal electrode 14 of acell having Ca²⁺ as the active ion may be less on an atomic basis thanabout 80%, 50%, 30%, 20% or 10%, with the balance being, e.g.,magnesium, lithium or sodium. The calcium concentration in the activemetal electrode 14 may be greater on an atomic basis than about 20%,40%, or 60%.

When the cell is fully charged (FIG. 3A), the molten active metalelectrode 14 of the illustrative calcium-bismuth battery 10 is a body ofabout 20 atomic percent calcium in magnesium (ρ_(liquid)=1.5 g/ml,T_(m)≈650° C.), and the alloy electrode 16 is a body of molten bismuth.After discharge (FIG. 3C), the active metal electrode 14 is relativelydepleted of calcium. The calcium missing from the active metal electrode14 has been transferred to the positive electrode 16, which has become abismuth-calcium alloy. The open-circuit voltage of the calcium-bismuthcell fully charged may be on the order of 1 V.

In another illustrative embodiment, referred to herein as amagnesium-antimony battery, the active alkaline earth metal of a battery50, shown in FIG. 4, is magnesium (ρ=1.5 g/ml, T_(m)=650° C.), and theadditional element diluting magnesium activity in the alloy electrode 16is antimony (ρ=6.5 g/ml, T_(m)=630° C.). The electrolyte 20 residingbetween the electrodes 14 and 16 comprises magnesium chloride. Themagnesium-antimony cell illustratively operates around 700° C. Thecontainer 22 and lid 26 are illustratively fashioned out of graphite.The insulating sheath 24 may be made of boron nitride. A metal plug,illustratively of tungsten, compression fit in the bottom of thecontainer 22 functions as the positive current collector 23. A moltensalt such as magnesium chloride in the electrolyte 20 more readily wetsthe graphite bottom of the container 22 than does a molten metal such asthe alloy electrode 16, thereby blocking electronic conduction betweenthe positive electrode 16 and the container 22. The metal plug securesan electronically conductive pathway between the molten positiveelectrode 16 and the positive terminal 30.

When the battery 50 is fully charged each of the electrodes 14 and 16 isits respective nominally pure liquid element, as shown for the battery10 in FIG. 3A. After discharge, the active metal electrode 14 in thebattery 50 (FIG. 4) remains monoelemental, but smaller in mass than whenthe cell 50 is charged, as shown for the battery 10 in FIG. 3C. Themagnesium missing from the active metal electrode 14 in the battery 50(FIG. 4) has been transferred to the positive electrode 16, which hasbecome an antimony-magnesium alloy. The alloying potential of magnesiumin antimony at 700° C. is on the order of 0.5 V.

The actual open-circuit voltage of, e.g., the calcium-bismuth ormagnesium-antimony cell, is influenced by the activities of the activealkaline earth metal in the electrodes, as expressed by the Nernstequation. The activities may exhibit large nonidealities which may shiftthe open-circuit voltage of the cell to values greater or less than itsexpected voltage. As mass of the active alkaline earth metal movesbetween the electrodes, changes in the respective chemical potentialschange the open-circuit cell voltage, so it is not constant over thecharge-discharge cycle.

In an alternative embodiment, the expense and complexity of electricallyinsulating the interior surface of the container 22 as shown for thebatteries 10 (FIG. 1) and 50 (FIG. 4) are eliminated by providing acurrent collector, in contact with the electrode layer disposed abovethe electrolyte 20, that isolates that electrode layer from thecontainer 22. With reference to FIG. 5A, in an alkaline earth metal ionenergy storage battery 60 an electronically conductive structure 62,illustratively fixed in position, comprises a shaft 62 a extendingoutside the lid 26 and constituting the negative terminal 28 of thebattery 60 and a contact portion 62 b, holding the liquid metal of thenegative electrode 14 away from the interior sides of the container 22and serving as the negative current collector 27. An insulating bushing64, illustratively of boron nitride or alumina, separates the shaft 62 aof the conductive structure 62 from the lid 26.

The structure 62 holds the active electrode 14 away from the container22, obviating the insulting sheath 24. With reference to FIG. 5B, duringdischarging, as the volume of the alloy electrode 16 increases, theelectrolyte 20 is pushed upward around the active alkaline earth metalelectrode 14. The structure 62 is configured so that some of the moltenelectrode 14 remains between the negative current collector 27 and theelectrolyte 20 when the cell is fully discharged and at all times.

Surface tension maintains the molten active-metal electrode 14 in placearound the contact portion of the structure 62. The contact portion maybe, e.g., mesh material folded into stacked layers or coiled into aspiral or tube. The mesh may be composed of strands on the order of 0.1to 1 mm in diameter, with similar spacing. Alternatively, the permeablecontact portion is a sponge.

Depending on the composition of the electrode 14, the structure 62 maybe made of, e.g., carbon, mild steel, or a steel alloy—containing, forexample, nickel and/or chromium—which is wet by the material ofelectrode 14. A wettable surface on the structure 62 promotes goodelectrical contact between the negative electrode 14 and its currentcollector 27. However, if material from the electrode 14 wetting theexterior of the contact portion 62 b breaks off and floats on thesurface of electrolyte 20 to the electrically conductive wall ofcontainer 22, the current-carrying efficiency of the battery 60 may bedegraded by unwanted reactions between the material of the electrode 14and the wall.

With reference to FIG. 6A, in another alternative embodiment, thenegative electrode layer 14 in a battery 70 is held in place above theliquid electrolyte 20 and away from the interior sides of container 22by an electrically conductive composite structure, shown generally at72, suspended from the lid 26.

The composite structure 72 comprises a shaft 72 a which extends upthrough an electrically insulating bushing 74 in the center of the lid26, the upper end of that shaft constituting the battery's negativeterminal 28. The bushing 74 may be of a suitable rigid, hightemperature-resistant material such as boron nitride or alumina. Theshaft 72 a is of a highly electrically conductive material such as steelor stainless steel that the material of the electrode layer 14 does wet.

The lower end of the structure 72 includes an inverted cup 72 b orcomparable cage, surrounding the shaft 72 a, that constitutes both thenegative current collector 27 and a containment for the electrode layer14. The cup 72 b is of a material such as mild steel that the electrodelayer 14 does not wet. Surface tension holds the electrode layer 14liquid material to shaft 72 a, but not to the cup. Thus, the structure72 may provide better containment of the electrode layer 14 material,keeping it away from the wall of the container 22, while ensuring goodelectrical connection between the negative current collector 27 and itselectrode layer 14.

Other composite collector/containment structures for the top electrodesimilar to the structure 72 may be envisioned for the electrode layer14. For example, the wettable shaft extension into the cup 72 b of thestructure 72 may be replaced by a ring 76 of the same material locatedjust inside the rim of the non-wettable containment cup as shown in FIG.6B or by a layer 78 of that same wettable material inside the top of thenon-wettable cup 72 b as shown in FIG. 6C.

In another alternative embodiment, the alkaline earth metal ion energystorage battery is configured for enhanced robustness by impeding mixingof the two electronically conductive liquids during shaking or tippingof the container 22. With reference to FIG. 7, in a reinforced battery80, an electrode separator 84 infiltrated by electrolyte is interposedbetween the active electrode 14 and the alloy electrode 16 and held byfriction to the sheath 24. The electrode separator 84 is illustrativelyof a material that is stable in contact with the molten electrolyte 20;wet by the molten electrolyte 20; and not wet by either of theelectrodes 14 and 16. The separator 84 is permeated with holes or otherporosity large enough to allow easy movement of ions between theelectrodes 14 and 16, but the surface tension relationships between theseparator 84 and the constituents 14, 16 and 20 of the cell 80 hindercontact between the negative 14 and positive 16 electrodes, therebydeterring shorting. The reinforced cell 80 may be constructed with acloser negative-positive electrode spacing, translating to less of theelectrolyte 20 and thus greater voltage efficiency, compared to a celllacking the separator 84.

When the active alkaline earth metal of the cell 80 is calcium, theseparator 84 is illustratively of alumina. Other suitable materials forthe electrode separator 84 may include ceramics such as magnesia,aluminum nitride, boron nitride, and silica glass. Illustratively, thepores in the separator are on the order of 1 to 5 mm in diameter.Depending on the surface tension values for the electrodes 14 and 16 andthe electrolyte 20, the pores may be larger or smaller.

The fixed separator 84 may be most appropriate for operating conditionsunder which the positions of the interfaces 42 and 46 move little, forexample a relatively short charge duration or charging at low currentdensity. If the illustrative cell charges or discharges at highcapacity, however, the interfaces 42 or 46 may move through the fixedseparator 84. For operation under these conditions, the cell 80 may beconstructed with a floating separator having a thickness less than orequal to the distance between the two interfaces 42 and 46.

Although conductive diffusion of molecules through liquids such as thoseconstituting the electrodes and the electrolyte of the illustrativebatteries is orders of magnitude faster than in solids, current throughthe all-liquid batteries may be mass-transfer limited due to relativelylarge diffusion distances in any of the layers 14, 16 and 20. Forexample, in a lithium-ion battery using micro- or nano-scale intercalantparticles, a diffusivity in the order of 10⁻¹² cm²/s is adequate forcomplete penetration of the Li⁺ ions at a rate that sustains chargingand discharging of the battery. By contrast, in the illustrativebatteries, diffusion distances may be millimeters or even manycentimeters. Thus, mass transport limitations may hamper proper functionof the illustrative batteries notwithstanding high diffusioncoefficients in the liquid electrodes 14 and 16 and in the liquidelectrolyte 20. For example, as a reactant in one of the electrodereactions is consumed, diffusion may not replace it at the respectiveelectrode/electrolyte interface at a rate that can support the cellcurrents made possible by the facile electrode reaction kinetics.

Inadequate mass transport in the illustrative batteries may furthermorespoil charging and discharging operations of the illustrative batteriesthrough other mechanisms. During charging of the illustrative alkalineearth metal ion battery as described above with reference to FIGS.2A-2C, active alkaline earth metal is driven from the alloy electrode 16across the alloy-electrolyte interface 46. Without adequate masstransport replenishing the region near the interface 46 from theinterior of the alloy electrode 16, the portion of the electrode 16reacting with the electrolyte 20 becomes metal-poor as chargingprogresses. As this depletion persists, the continuing operation of thecharging circuit 48 may provoke other, undesirable electrode reactionsat the interface 46.

Likewise, the desired electrode reactions, prescribed above, may beinhibited by the concentration of reaction products near anelectrode/electrolyte interface. In the case of the illustrativealkaline earth metal ion battery, discharging relies on disparateactivities of the alkaline earth metal at the respectiveelectrode/electrolyte interfaces, described above with reference toFIGS. 3A-3C. During movement of the active alkaline earth metal from thenegative electrode 14 to the alloy electrode 16, as the concentration ofthe active metal reaction product increases in the alloy electrode 16 atthe alloy-electrolyte interface 46, the driving force of theelectrochemical cell reaction moving the active alkaline earth metalinto the alloy electrode 16 decreases. If the active alkaline earthmetal in the alloy electrode 16 is located disproportionately near theinterface 46, so that the concentration at the interface 46 does notreflect that electrode's global composition, the voltage delivered bythe illustrative battery is compromised compared to what would bepossible with a uniform electrode composition. For sufficient localconcentrations of the active alkaline earth metal near the interface 46,discharging of the battery may cease altogether.

Accordingly, mass transport mechanisms other than conductive diffusioncontributing to homogenization of the compositions of the liquid layers14, 16 and 20 during charging and discharging may be valuable inachieving optimum operation of the illustrative batteries. By contrast,in a conventional high-temperature electrochemical metal extractionsystem, electroreduction augments the metal content of a substantiallyliquid metal body, in which concentration gradients are not operative.Thus, with intra-metal mass transport being relatively inconsequential,such processes may actually be configured to minimize movement withinliquid layers in order to avoid shorting.

Alternative embodiments described hereinbelow are configured to enhancetransport of active species to one or both electrode/electrolyteinterfaces by generating convective flow within the liquid materiallayers in a battery such as, e.g., an alkaline earth metal ion battery.Transport-enhancing features function to induce flow within one or moreof the liquid layers 14, 16 and 20, such as by generating one or morebuoyancy- or gravity-driven or magnetically induced convection orcirculation cells, which may cause mixing of the liquid material in oneor more of the layers 14, 16 and 20 and convey material to and fromrespective electrode/electrolyte interfaces. While approaches totransport enhancement are described herein specifically in the contextof high-temperature, liquid-electrode batteries, the enhancementsdescribed may also be useful in other electrochemical systems havingliquid components, for example in selected electrowinning systems orlower-temperature devices such as, e.g., a fuel cell.

The flow induced in the liquid constituent(s) of the illustrativestorage device does not have to be very fast to provide enhancedtransport of species to and from the electrodes/electrolyte interface(s)and significantly enhance battery productivity. In fact, it can be shownthat with a diffusivity of 10⁻⁵ cm²/s in a liquid, a liquid flow rate ofonly ˜0.1 mm/s provides more active species at the electrode/electrolyteinterface than that caused by diffusion by itself in the liquid.Illustratively, the present storage device should produce a flow rate inthe range of 0.1 to 1.0 mm/s.

In one approach to inducing flow in the illustrative batteries, thecirculation producer produces or develops a thermal gradient in at leastone of the liquid constituents 14, 16 and 20. The resultingnonuniformity in density may generate gravity or buoyancy-drivenconvective flow cells, sometimes referred to as Rayleigh-Bènard cells,in the liquid constituent. These initial free convection cells may, inturn, induce similar circulation in an adjacent constituent resulting inmixing of some, if not all, the liquid constituents of the battery. Thecirculation producer may include various different thermal flowmanagement devices to initiate one or more free convection cells in atleast one of the electrode or electrolyte layers of the battery toachieve the stated objectives. The battery may be configured to exploitthe thermal energy present therein during normal operation, e.g., theheat that maintains the battery's constituents in a molten state or thatis generated from joule heating of the battery by thecharging/discharging thereof. In another embodiment, the battery mayincorporate additional sources of heat.

A thermally insulating housing, enclosing the container 22, may formpart of a circulation producer. The circulation producer furthermoreincludes one or more thermal management devices in a wall of theinsulation. The thermal management device may be configured to provide aheat transfer path so that heat may be conducted away preferentially orasymmetrically from at least one of the liquid constituents 14, 16 and20 of the battery. The resulting thermal gradient in the constituentcreates free or gravity-driven convective flow within that constituent.Thus enhanced mass transport is achieved between the electrodes 14 and16 without the cost and complexity of a pumping system effecting forcedconvective flow, such as is used in flow cells, for example.

Thus, with reference to FIG. 8, in an illustrative embodiment, a battery90 incorporates thermal management devices 98 in the form of metal rodsextending through a thermally insulating housing 96 to the oppositesides of the container 22 at the level of electrolyte layer 20 therein.The devices 98 are in intimate thermal contact with the conductive wallsof container 22 so that, in effect, the container is less insulated atthose locations. The devices 98 provide a heat transfer path between thecontainer 22 and an outside space. Therefore, the liquid electrolyte 20near the devices 98 is cooler, and therefore more dense, than at thecenter of the battery 90, causing liquid material in the electrolyte 20to sink at those locations. Thus, the dissipation of heat (Q) viacontainer 22 creates one or more convection cells in the electrolytelayer 20 as indicated by the circular arrows shown in phantom in FIG. 8.Illustratively, the connection of the positive terminal 30 to container22 is located above the negative electrode 14 as shown to minimize heatdissipation via that electrode. In this case, the induced temperaturegradient may be controlled solely by the thermal management devices 98.

Once the convection cells have been established in the layer 20, theinterfacial boundary condition between it and the liquid layer 14 above,and the liquid layer 16 below, may cause movement in those layers,giving rise to similar circulation in layers 14 and 16 as indicated bythe circular arrows in those layers. Thus, the flow induced in eachlayer in container 22 may introduce fresh reactive material to andconvey products from the interfaces between those layers, therebypromoting the desired electrochemical reaction in the battery 90.

FIG. 9 shows another embodiment, similar to the battery 90 shown in FIG.8 except that the thermal management devices 98 (e.g., metal rods) arepresent in the housing 96 at the level of the one of the electrodelayers that is disposed under the electrolyte 20. Illustratively, in thealkaline earth metal ion battery, the positive electrode layer 16 isunder the electrolyte 20 at the bottom of the container 22. Since theFIG. 9 battery includes the same components and operates in more or lessthe same way as the battery 10 in FIG. 8, the in-common componentsthereof bear the same identifying numerals. Also, for ease ofillustration, the terminals 28 and 30 (FIG. 1) have been omitted fromFIG. 9 and subsequent drawing figures.

In a manner similar to that occurring in battery 90 of FIG. 8, the heatremoved from the sides of the positive electrode layer 16 via the sidewalls of container 22 and the devices 98 produces a thermal gradienttherein which causes convection of the liquid material thereof asindicated by the circular arrows shown in phantom in FIG. 9. This mayincrease the flux within the electrode 16 of components to and away fromthe interface between the layers 16 and 20, thereby promoting desiredelectrochemical reaction thereat. Since the positive electrode layer 16,illustratively being of a metal or metalloid, is more dense thanelectrolyte layer 20, e.g., salt, this embodiment may require a largerthermal gradient to develop the initial convection cells in electrodelayer 16 than is the case for the electrolyte layer 20 of the device inFIG. 8.

Although not shown in FIG. 9, the initial convection cells in theelectrode layer 16 may induce flow or circulation in the adjacentelectrolyte layer 20, and so on into the electrode layer 14 in a mannersimilar to that shown in FIG. 8.

FIG. 10 illustrates a battery 90 which is essentially the same as thedevice in FIG. 9, except that it is longer or deeper. In this case, thethermal management devices 98 are spaced along the housing 96 anddesigned so that heat is dissipated via the side walls of container 22all along the container to encourage the development of elongatedcylindrical convection cells in electrode layer 16 as shown by thecylindrical arrows in FIG. 9.

Instead of providing individual heat dissipation devices 98 at each sideof housing 96 as shown in FIGS. 8-10, devices 98 in the form of platesmay be used, those plates being designed and dimensioned to produce therequired temperature gradient in the operative liquid constituent tocause convective flow thereof.

FIG. 11 illustrates a battery similar to the battery shown in FIG. 9wherein the interior bottom wall of container 22 is formed withspaced-apart cusps 22 a whose spacing promotes the formation of stableconvection cells of a determined size in the electrode layer 16. As inthe previous storage devices 90, these initial convection cells maypromote similar circulation of the liquid material in the overlyingliquid layer 20.

FIG. 12 shows a battery of cylindrical geometry having a thermallyinsulating housing 96 and a single thermal management device 98 thereinin the form of a metal ring at the level of the positive electrode layer16. In this embodiment, heat is dissipated radially from the interior ofthe device via the container 22 and device 98 all around the verticalaxis of the battery 90 so that a convection cell in the form of a torusis formed in electrode layer 16. As in the earlier describedembodiments, this convective flow in electrode layer 16 may inducesimilar circulation in the adjacent liquid layer 20 in container 22.Also, the ring could be located at the level of layer 14 or 20 to inducesuch convective flow therein.

FIG. 13 illustrates another battery 90 similar to the one in FIG. 9wherein a single thermal management device 98, e.g. a metal rod, islocated at only one side of housing 96 at the level of one of thebattery's liquid constituents, electrode layer 16 in this instance. Thisasymmetric removal of heat from the battery 90 still sets up gravity- orbuoyancy-driven convection in the operative constituent, i.e., theelectrode 16, as indicated in that figure. In fact, a thermal gradientmay be produced in one or more of the battery's liquid constituents byemploying a thermal management device 98 which includes a portion of thewall of the housing 96 that is thinner and/or has a smaller thermalconductivity at one side of container 22 than at another portion of thehousing 96, such as another side. The liquid layer 16 on the lessinsulated side of the container 22 would then be cooler, and thereforemore dense, than the liquid elsewhere in the container, which wouldcause it to sink, thereby promoting free convective mixing of the liquidmaterial in the layer 16 as shown by the circular arrows in FIG. 13.

Refer now to FIG. 14, which illustrates an energy storage device orbattery 90 wherein heat is extracted or dissipated from the contents ofthe container 22 via the device's lid 26 and negative current collector27. In this case, a thermal management device 98, e.g., a metal rod orplate, extends through one side of the insulating housing 96 and is incontact with the lid 26. The lid 26 is in contact with the one of theelectrodes which is disposed over the electrolyte 20, near the top ofthe container 22, illustratively the negative electrode 14. Heat (Q) isdrawn from the electrode layer 14 via the lid 26 including its negativecurrent collector 27 and the device 98. This creates a thermal gradientin the electrode layer 14 which creates free convection cells therein.These may, in turn, induce similar flow in the underlying electrolytelayer 20 as shown by the circular arrows in FIG. 13.

Turn now to FIG. 15, which shows a battery 90 wherein the thermalmanagement device 98 introduces heat into one of the liquid constituentsof the battery, herein the positive electrode layer 16, to supplementheat therein. In this embodiment, device 98 includes a heating element102 in the bottom wall of the container 22 energized by leads extendingthrough the bottom wall of the housing 96 to an external current source104. Heat is dissipated through one or more of the walls of the housing96 to promote the creation of convection cells in the electrode 16 asshown.

In the illustrative embodiments of the battery 90 shown, the convectioncells created in one or another of the battery's liquid constituents arebuoyancy- or gravity-driven convection cells caused by a thermalgradient produced by controlled management of thermal energy present inthe battery.

In another approach to enhancing transport of reactive species orproducts in the illustrative batteries, magnetic induction caused by thecurrent flowing when the battery is being charged or discharged inducesflow in one or more of the liquid constituents. This type of circulationproducer creates a current path to at least one of the currentcollectors 23 and 27 that gives rise to a magnetic field around oradjacent to that collector. The magnetic field produced coacts with thecurrent in the electrode layer in contact with that collector to producestirring force therein which circulates the liquid material of thatlayer. This circulation of liquid material may introduce material to andconveys material away from the associated electrode/electrolyteinterface, thus enhancing the battery's current density and/or promotingdesired electrochemical reaction. Various different current collectordesigns are disclosed which promote such circulation.

FIG. 16 illustrates a battery 100 incorporating a circulation producercomprising a magnetic induction device 103 in the form of a protrusion105, for example a bulge or ridge, that protrudes from the lid 26 downinto its electrode, i.e., the electrode layer disposed over theelectrolyte 20, e.g. near the top of the container 22. Illustratively,the top electrode layer is the negative electrode 14. Thus, in thiscase, the protrusion 105 also constitutes the negative current collector27. Again, the components of the battery 100 shown in FIG. 16 that arecomparable to those in the battery embodiments depicted in FIGS. 8-15bear the same identifying numerals.

When the battery 100 is being charged by an external power source (notshown) connected to the battery's positive 30 and negative 28 terminals(FIG. 2), electrons flow from the charging source via the lid 26 and itsprotruding negative current collector 27, 105 into the negativeelectrode layer 14. The protrusion 105 is shaped so that the current (I)therethrough produces an azimuthal magnetic field B more or lesscentered on the vertical axis of the protrusion and follows a divergentpath into the electrode layer 14. The interaction of the magnetic fieldB with the horizontal component of the divergent charge carrier flow Iin the electrode layer 14 produces a stirring force ({right arrow over(F)}=q({right arrow over (V)}×{right arrow over (B)})) in the electrodelayer that causes the development of one or more circulation cellstherein as indicated by the circular arrows in FIG. 16. This circulationmay bring reactive material from the interior of the electrode 14 to itsinterface with electrolyte layer 20 and convey interface material to theinterior as described above.

As in the other battery embodiments, the circulation in the layer 14may, in turn, induce circulation of the underlying layer.

When the battery 100 is connected to an external load (not shown) and isdischarging, the current flows in a reverse direction from that shown byarrows I in FIG. 16, converging into protrusion 105, creating a similarcirculation of the liquid material in the electrode layer 14 thatproduces a similar effect.

FIG. 17 illustrates a similar battery 100 wherein circulation cells arepromoted in the electrode layer disposed under the electrolyte 20, e.g.,at the bottom of the container 22, by the configuration of the electrodelayer's respective current collector. Illustratively, the layer disposedunder the electrolyte 20 is the battery's positive electrode layer 16.An induction device 103 in the form of a protrusion 105, such as a bulgeor ridge, in the positive current collector 23 extends into the positiveelectrode 16. Here, the floor of container 22 is covered by anelectrically insulating layer 107 that has a central opening 107 a toprovide clearance for the protrusion 105 and to confine the current flowthereto. The current through that protrusion 105 produces a magneticfield therearound which interacts with the divergent or convergentcurrent flow in the layer 16 when the battery 100 is being charged ordischarged to promote circulation of the liquid material in theelectrode layer 16 in a manner similar to that produced in the electrodelayer 14 of the battery 100 shown in FIG. 16.

In some applications, the magnetic induction devices in the batteries100 depicted in FIGS. 16 and 17 may be combined in a single battery topromote circulation in both of the electrode layers 14 and 16 at thesame time.

In FIG. 18, another battery embodiment 110 is depicted which producescirculation cells by magnetic induction in the electrode layer disposedover the electrolyte 20, e.g., near the top of the container 22.Illustratively, the electrode disposed over the electrolyte 20 is thenegative electrode layer 14 of the battery 110. In this embodiment, thebattery 110 has a circulation producer comprising a magnetic inductiondevice 103 comprising a negative current collector having a more or lesscylindrical protrusion 114 that extends down from cap 112 verticallyinto the electrode 14 at an off-center location in the container 22.Also, a negative terminal 116 is provided which has an upper endconnected to the cap 112 and extends down vertically close to the sidewall of the container 22, substantially parallel to the protrusion 114.The free, lower end of that terminal 116 is adapted to be connected tothe positive terminal of a similar battery or other energy-exchangingdevice.

During a charging cycle, when electrons flow along the terminal 116 inthe direction of arrows I to the protrusion 114 and into the electrode14, a magnetic field B, the flux lines of which extend into thecontainer 22 as shown in the drawing, is produced around the terminal116. The magnetic field B interacts with the electrons flowing from theprotrusion 114 into the electrode layer 14, producing a verticalstirring force F in that electrode which may circulate fresh material toand from the interface of the electrode 14 with the electrolyte layer 20as described above. When the storage device 110 is discharging, with thecurrent flowing in the reverse direction along the protrusion 114 andthe terminal 116, similar circulation cells are formed in the layer 14.

The alkaline earth metal ion cell 10 (FIGS. 1-3), 50 (FIG. 4), 60 (FIGS.5A and 5B.), 70 (FIG. 6) or 80 (FIG. 7), especially when equipped withcirculation producing components such as shown in any of the batteries90 (FIGS. 8-15), 100 (FIG. 16-17) or 110 (FIG. 18) may be capable ofrapidly receiving and dispatching electricity, thereby bridging asupply-demand mismatch. The illustrative energy-storage cells mayoperate at extreme temperatures, such as arctic cold and desert heat,without restriction on geographical location and are realizable in amobile structure. The power capacity is large, on the order of 10 m²/MW,and scalable for adaptation to a variety of large-scale and commercialpower management applications.

Several approaches are possible in expanding the capacity of thealkaline earth metal ion energy storage cell to adapt it to therequirements of large-scale applications, on the order of several MW. Inone approach, scalability may be exploited in a single large alkalineearth metal ion energy storage battery unit by increasing the mass ofthe electrodes 14 and 16 and thereby increasing the mass of alkalineearth metal available for transfer within the cell. In another approach,a battery including many smaller alkaline earth metal ion unitsconnected in series may confer a higher battery voltage more practicallyintegrated with the power electronics necessary to serve on large-scalesystems. In yet another approach a large array of units may beinterconnected with series and parallel connections for increasedrobustness with respect to failure due to individual cell malfunction.

In one embodiment, a single alkaline earth metal ion battery unit 10 ofthe type shown in FIG. 1 is used to make a battery of more usablevoltage in the following way. FIG. 19 shows in perspective view the cell10 of the configuration type shown in FIG. 1. The cell 10 illustrativelyis a cube 10 cm long on each side. FIG. 20 shows a linear assembly 120formed of four such battery units 10 connected in series. In FIG. 21,four linear assemblies 120 are joined to form an array 122 of 16 units10 connected in series, in which the direction of electron movementduring charging is indicated by arrows 124. Such arrays areillustratively stacked and electrically joined six high into modules of96 cells to create a battery having an open-circuit voltage on the orderof 100 V.

One potential use for the alkaline earth metal ion energy storagebattery is at a large-scale power generator. The diurnal fluctuation inenergy demand reduces plant efficiency, thereby increasing emissions bypreventing generator operation at optimum output levels around theclock. A high-capacity electrical energy storage apparatus, with a powercapacity greater than 1 MW, could allow load-leveling, which is effectedby downloading power from the generator to a storage device duringlow-demand periods and then uploading power to the grid during times ofhigher demand, permitting the power plant to operate at a constantlevel.

A second potential use for the alkaline earth metal ion energy storagebattery is at renewable energy source converters. Variability in supplymakes management of power generated by renewable sources challenging.Sources such as wind and solar energy generate only intermittently.Without adequate power storage, additional power generators are neededon standby to operate in the event that the wind stops blowing or thesky clouds over. The underutilized capital in the form of excess powergenerators ultimately may limit the scale of deployment of renewableenergy sources. A reliable high-capacity electrical storage device usedin conjunction with a renewable energy source could provide dedicatedload leveling thereby supporting implementation of renewable energysources on grid. Such a combination could also support the use ofintermittent renewable energy sources as an alternative to generators inremote, off-grid locations to which periodic delivery of fuel would bedifficult.

A third potential use for the alkaline earth metal ion energy storagebattery is in support of transmission lines. Transmission anddistribution systems generally have no storage capacity, so the gridmust meet instantaneous demand. As the load on a transmission lineapproaches its capacity, it incurs heavy ohmic losses which decrease itsefficiency. Furthermore, the resulting resistive heating can melt systemcomponents and cause transmission line failure. Portable generators ofthe requisite power capacity (tens of MW) available to boost supply atthe load center may be noisy, polluting, and require periodic refueling.Upgrading or replacing transmission lines as they reach capacity limitsis very expensive and frequently meets with public opposition.Construction can take as long as five years.

A re-locatable alkaline earth metal ion energy storage unit located neara load center could supply a portion of the energy carried by thetransmission line during peak hours of the day, thereby mitigating loaddemands on the line. Ideally, the storage unit would provide asignificant portion, say at least 2% to 20% of the line's capacity,which is typically on the order of 500 MW. Such a unit could defer theneed for a transmission line upgrade. Or, a portable alkaline earthmetal ion energy storage unit could be deployed to supply emergencypower after a system failure or to maintain power delivery duringconstruction of new lines and then be relocated when no longer needed.

Distribution systems from load centers suffer similar problems, albeitat much lower loads, and could be similarly addressed using a portablepower storage unit. Commercial consumers requiring a constant supply ofelectricity are especially vulnerable to blackouts. Auxiliary generatorsare less than ideal for backup because they require time to reach fulloutput levels. These consumers would benefit from backup power systems,or uninterruptible power systems (“UPS”) configured to provideelectricity to such a facility in the event of a grid-power failure. Acharged alkaline earth metal ion energy storage unit, configured todischarge when the power is interrupted, could function in that role.

Finally, a facility that is sensitive to voltage irregularities can beadversely affected by brownouts or other inconsistencies in deliveredpower. A UPS in the form of a charged alkaline earth metal ion energystorage unit, configured to discharge to eliminate deviations from thedesired power level, could act as a buffer between the grid and thefacility to ensure high power quality.

Although specific features of the invention are included in someembodiments and drawings and not in others, it should be noted that eachfeature may be combined with any or all of the other features inaccordance with the invention.

It will therefore be seen that the foregoing represents a highlyadvantageous approach to energy storage, e.g., for large-scale andcommercial energy management. The terms and expressions employed hereinare used as terms of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof, butit is recognized that various modifications are possible within thescope of the invention claimed.

What is claimed is:
 1. An energy storage device configured to exchangeenergy with an external device, the energy storage device comprising: acontainer having an electrically conductive portion; a lid that coversthe container; a negative electrode disposed away from the container; apositive electrode in contact with the electrically conductive portionof the container; an electrolyte contacting the negative electrode andthe positive electrode at respective electrode/electrolyte interfacesand contacting the electrically conductive portion of the container,wherein at least two of the negative electrode, the positive electrode,and the electrolyte comprise separate liquid materials within thecontainer at an operating temperature of the energy storage device. 2.The energy storage device of claim 1, wherein, during charge/discharge,a thickness of the electrolyte remains substantially constant.
 3. Theenergy storage device of claim 1, wherein the energy storage device hasa power capacity greater than about 1 MW.
 4. The energy storage deviceof claim 1, wherein the energy storage device comprises one or morecells, and wherein an individual cell comprises the negative electrode,the positive electrode and the electrolyte.
 5. The energy storage deviceof claim 1, wherein (i) the negative electrode comprises calcium,magnesium, or a mixture thereof, (ii) the positive electrode includes amaterial selected from the group consisting of tin, lead, bismuth,antimony, tellurium, selenium, and combinations thereof, or (iii) thenegative electrode comprises calcium, magnesium, or a mixture thereof,and the positive electrode includes a material selected from the groupconsisting of tin, lead, bismuth, antimony, tellurium, selenium, andcombinations thereof.
 6. The energy storage device of claim 1, furthercomprising a structure configured to hold the negative electrode awayfrom the walls of the container.
 7. The energy storage device of claim6, wherein the structure comprises a contact portion comprising a meshmaterial folded into stacked layers, a mesh material coiled into aspiral, a mesh material coiled into a tube, a sponge, a cup, or a cage.8. The energy storage device of claim 6, wherein the structure comprisesa negative current collector, and wherein the structure is configured sothat the negative electrode remains between the negative currentcollector and the electrolyte when the cell is fully discharged and atall times.
 9. The energy storage device of claim 6, wherein thestructure extends away from the lid in a direction that is substantiallyperpendicular to the lid.
 10. An energy storage device, comprising: acontainer; a lid that covers the container; a negative electrode; apositive electrode; and an electrolyte contacting the negative electrodeand the positive electrode at respective electrode/electrolyteinterfaces, wherein the electrolyte is liquid at an operatingtemperature of the energy storage device, wherein the negativeelectrode, the positive electrode, or both are liquid at the operatingtemperature of the energy storage device, and wherein the negativeelectrode comprises an active alkaline earth metal and at least oneadditional negative electrode metal that is present at an amount that(i) decreases a melting point of the negative electrode or (ii) reducesa thermodynamic activity of the active alkaline earth metal in thenegative electrode, wherein the at least one additional negativeelectrode metal include magnesium present at an amount that decreasesthe melting point of the negative electrode while diminishing thevoltage of the cell by no more than about 0.1 V.
 11. The energy storagedevice of claim 10, wherein each of the negative electrode, positiveelectrode and electrolyte is liquid at the operating temperature. 12.The energy storage device of claim 10, wherein: (a) the energy storagedevice is coupled to (i) a power generator, (ii) an intermittentrenewable energy converter, (iii) a load center on a transmission line,or (v) a distribution system coupled to a transmission line; or (b) theenergy storage device provides (i) load leveling, (ii) re-locatablepower supply capacity coupled to a transmission line, (iii) backup oruninterruptible power for a load coupled to a distribution system, or(iv) power buffering between an electrical grid and a load.
 13. Theenergy storage device of claim 10, wherein, during operation, thepositive electrode comprises the active alkaline earth metal and atleast two additional positive electrode elements, and wherein at leastone of the at least two additional positive electrode elements reduces athermodynamic activity of the active alkaline earth metal.
 14. Theenergy storage device of claim 10, wherein, during operation, thepositive electrode comprises the active alkaline earth metal and atleast two additional positive electrode elements, and wherein at leastone of the at least two additional positive electrode elements is tin,lead, bismuth, antimony, tellurium or selenium.
 15. The energy storagedevice of claim 10, wherein, during operation, the positive electrodecomprises the active alkaline earth metal and at least two additionalpositive electrode elements, and wherein the active alkaline earth metal(i) alloys with at least one of the at least two additional positiveelectrode elements upon discharge, (ii) de-alloys from at least one ofthe at least two additional positive electrode elements upon charge, or(iii) both alloys with at least one of the at least two additionalpositive electrode elements upon discharge and de-alloys from at leastone of the at least two additional positive electrode elements uponcharge.
 16. The energy storage device of claim 10, wherein duringoperation, the positive electrode comprises the active alkaline earthmetal and at least two additional positive electrode elements, andwherein at least one of the at least two additional positive electrodeelements is present at an amount that adjusts a melting point of thepositive electrode.
 17. The energy storage device of claim 10, whereinthe active alkaline earth metal is calcium.
 18. The energy storagedevice of claim 10, wherein a concentration of the active alkaline earthmetal in the negative electrode is (i) greater than about 20% on anatomic basis, (ii) less than about 80% on an atomic basis, or (iii)greater than about 20% on an atomic basis and less than about 80% on anatomic basis.
 19. The energy storage device of claim 10, wherein theactive alkaline earth metal is calcium, and wherein the electrolytecomprises a halide salt of calcium in an amount from about 5 mol % toabout 50 mol %, wherein said halide salt of calcium conducts calciumfrom the electrolyte to the positive electrode or from the positiveelectrode to the electrolyte.
 20. The energy storage device of claim 10,wherein the electrolyte comprises a salt of the active alkaline earthmetal and a supporting electrolyte salt that suppresses dissolution ofthe active alkaline earth metal from the negative electrode into theelectrolyte, and wherein the supporting electrolyte salt isligand-donating.
 21. The energy storage device of claim 10, wherein theelectrolyte has an electrical conductivity of at least about 0.01siemens/cm.
 22. The energy storage device of claim 10, wherein theelectrolyte comprises a mixture of a halide salt of the active alkalineearth metal and a halide salt of an alkali metal.
 23. The energy storagedevice of claim 22, wherein the electrolyte comprises a mixture ofcalcium chloride with a halide salt of potassium or sodium, and whereinthe halide salt of potassium or sodium comprises a chloride, an iodideor a bromide salt of potassium or sodium.
 24. The energy storage deviceof claim 10, wherein the operating temperature is less than about 750°C., and wherein the melting point of the negative electrode is less thanor equal to the operating temperature.
 25. The energy storage device ofclaim 24, wherein the operating temperature is greater than about 300°C. and less than about 700° C.
 26. The energy storage device of claim10, wherein each of the negative electrode, the positive electrode andthe electrolyte includes the active alkaline earth metal when the energystorage device is not fully charged, and wherein the positive electrodeis nominally free of the active alkaline earth metal when the energystorage device is fully charged.
 27. The energy storage device of claim26, wherein the active alkaline earth metal is present in an elementalform in the negative electrode, an alloy form in the positive electrodeand a salt in the electrolyte, and wherein the electrolyte comprisescations of the active metal.